It is often necessary or desirable to determine the location of a medical probe relative to a location of interest within three-dimensional space. In many procedures, such as interventional cardiac electrophysiology therapy, it is important for the physician to know the location of a probe, such as a catheter, (especially, a therapeutic catheter) relative to the patient's internal anatomy. During these procedures, a physician, e.g., steers an electrophysiology (EP) mapping catheter through a main vein or artery into the interior region of the heart that is to be treated. The physician then determines the source of the cardiac rhythm disturbance (i.e., the targeted cardiac tissue) by placing mapping elements carried by the catheter into contact with the heart tissue, and operating the mapping catheter to generate an EP map of the interior region of the heart. Having identified the targeted cardiac tissue, the physician then steers an ablation catheter (which may or may not be the same catheter as the mapping catheter above) into the heart and places an ablating element carried by the catheter tip near the targeted cardiac tissue, and directs energy from the ablating element to ablate the tissue and form a lesion, thereby treating the cardiac disturbance.
Traditionally, navigation of catheters relative to points of interest has been accomplished using fluoroscopy. In this case, radiopaque elements are located on the distal end of the catheter and fluoroscopically imaged as the catheter is routed through the body. As a result, a two-dimensional image of the catheter, as represented by the illuminated radiopaque elements, is generated, thereby allowing the physician to roughly determine the location of the catheter. The use of fluoroscopy in locating catheters is somewhat limited, however, in that the physician is only able to visualize the catheter and surrounding tissues in two dimensions. In addition, fluoroscopy does not image soft tissues, making it difficult for the physician to visualize features of the anatomy as a reference for the navigation. Thus, fluoroscopy is sub-optimal for the purpose of navigating a catheter relative to anatomical structure composed primarily of soft tissues, e.g., within the heart.
Various types of three-dimensional medical systems (e.g., the Realtime Position Management™ (RPM) tracking system, developed commercially by Boston Scientific Corporation and described in U.S. Pat. No. 6,216,027 and U.S. patent application Ser. No. 09/128,304, entitled “A Dynamically Alterable Three-Dimensional Graphical Model of a Body Region,” and the CARTO EP Medical system, developed commercially by Biosense Webster and described in U.S. Pat. No. 5,391,199) have been developed, or at least conceived, to address this issue. In these medical systems, a graphical representation of the catheter or a portion thereof is displayed in a three-dimensional computer-generated representation of a body tissue, e.g., a heart chamber. The three-dimensional representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a three-dimensional coordinate system, e.g., by moving a mapping device to multiple points on the body tissue. The position of the device to be guided within the body tissue is determined by placing one or more tracking elements on the device and tracking the position of these elements within the three-dimensional coordinate system.
In the RPM tracking system, this is accomplished by moving the mapping device within the heart chamber to acquire a volume of interior anatomical points (i.e., points within the blood pool) and deforming a graphical anatomical surface to be coincident with the outermost interior anatomical points as each anatomical point is acquired. The anatomical surface can be made more accurate by touching the endocardial surface with the mapping device to acquire anatomical surface points and tying the anatomical surface to these points. In the CARTO EP medical system, a multitude of anatomical surface points are acquired, and once a sufficient number is acquired, a graphical anatomical surface is created based on the surface points.
In both of the RPM and CARTO EP systems, once the graphical heart representation has been created, an EP mapping catheter, which includes at least one tracking element, so that it can be tracked within the three-dimensional coordinate system, is used to acquire EP information along the endocardial surface. An electrical activity map can then be generated from the acquired EP information and superimposed over the graphical heart representation. An ablation catheter, which like the EP mapping catheter, includes at least one tracking element, so that it can be tracked within the three-dimensional coordinate system, is placed into contact with the targeted treatment regions identified in the electrical activity map and operated to therapeutically ablate the tissue.
While the RPM and CARTO EP systems have generally been successful in providing a means for navigating catheters within anatomical structures, the anatomical information acquired by the mapping device at any given moment is represented by a single point. As such, many anatomical measurements must be made to create a relatively accurate graphical reconstruction of the anatomical structure, thereby increasing the time required to perform the relevant medical procedure. In addition, when creating anatomical surface points, the accuracy of the resulting graphical model will depend upon whether or not contact between the mapping device and the surface of the anatomical structure has actually been made during acquisition of the surface points. However, it is difficult to ensure that such contact is always made, thereby resulting in some inaccuracies within the graphical model. It is also sometimes difficult to determine when the EP mapping catheter and ablation catheter are in contact with the endocardial surface during the EP mapping and ablation functions, thereby making the medical procedure more tedious.
There thus remains a need for an improved system and method for generating graphical representations of anatomical structures and navigating medical devices within such anatomical structures.